This invention was made in the course of, or under, a contract with the Energy Research and Development Administration.
Large plasma containment devices such as tokamaks are of considerable research interest at the present time in the quest for practical fusion power. One such device is the ORMAK described in the ORNL Thermonuclear Division Report, ORNL-4688, dated August, 1971, and another such device is the ORMAK F/BX described in the ORNL Thermonuclear Division Report, ORNL-TM-4634, issued on July 11, 1974. In such devices, a fraction of the normal plasma energy is expected to leave the plasma in the form of radiation and energetic charge exchange neutral particles and, in some cases, neutrons. This energy is deposited more or less uniformly over the containment wall.
Energy also leaves the plasma in the form of charged particles, ions and electrons. These arrive at the surface of the plasma by plasma convection and conduction processes. Due to the properties of the magnetic field which serves to limit the loss of charged particles, the natural shape of the plasma is a toroid. The minor diameter of this toroid is determined by the "scraping off" of outward-diffusing plasma on the first material surface encountered.
In any typical installation, such as in ORMAK and other existing tokamaks, a poloidal ring limiter is positioned at one azimuthal location around the major axis of the plasma. The basic purpose of this or any limiter is to intercept energetic particles before they can impinge on the containment walls, thus preventing any localized damage which would occur if the most inward point of the containment wall were allowed to limit the plasma size. Variations of the basic poloidal ring limiter have also been recently employed. These are all derived from the understanding that the limiter need not be continuous but could consist of tabs, blades, or rails at one or more locations around the plasma.
In the normal operation of a large toroidal plasma confinement device, the power flux to a conventional poloidal ring limiter may turn out to be severe such that the limiter would function somewhat inadequately due to the intense local heating thereof.
Thus, there exists a need for a more efficient plasma limiter means and/or arrangement of such a means to ensure that the plasma energy is distributed as uniformly as possible over the surface of such a means. The present invention was conceived to meet this need in a manner to be described hereinbelow.